It is known that olefin polymers may be modified by reacting oxidized olefin polymer with a methylolated phenolic material, U.S. Pat. No. 3,211,804. In the known process, it is essential that the olefin polymer is oxidized to increase the carbonyl or peroxide content of the olefin polymer prior to reaction with the methylolated phenolic material. A disadvantage when using milder oxidizing agents such as air, is that long reaction times, i.e., 45 minutes or more, are required. Disadvantages when using peroxide as the oxidizing agent are that excessive crosslinking can result and polymer degradation can occur through chain scission especially with branched chain polymers such as polypropylene. Thus, it is desirable to provide olefin polymers modified with methylolated phenolic material without pre-oxidizing the olefin polymer.